The present invention relates to a method for estimating a torque of a three-phase driver motor for a vehicle, which is supplied with power by a converter, with three phase lines leading from the converter to the drive motor. Additionally the present invention relates to a corresponding drive device for a vehicle.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
In electrical traction drives (e.g. in the case of an electric car) it must be ensured, in order to safeguard track stability, that the target torque defined by a vehicle controller is largely correctly converted as an actual torque on the wheel. By comparing target and actual values it is possible to infer that the drive is working free from errors and consequently that track stability is being complied with. Determining the torque is thus an important safety feature. The actual torque should therefore be determined using operational equipment (a torque monitor) which is independent of the control branch in which target values are supplied by control electronics.
Whereas the target value of the torque is known (directly available from the control algorithms of a vehicle controller), the actual torque can only be directly measured with considerable effort. Standard commercial torque measuring shafts cannot be used in a vehicle for reasons of space, weight and cost. Therefore instead of a torque measurement it is necessary to use estimation or calculation methods based on variables that can be measured more easily.
Three-phase motors or alternating current motors are typically used for the electrical drive of vehicles. Such an alternating current motor is normally controlled by an intermediate circuit via a converter (see FIG. 1). The converter is controlled by control electronics or a controller.
As long as the converter is pulsing and the controller is working, it is possible to use the actual current values and the target voltage values to calculate the torque. A precondition for this is that the target voltage values are converted tolerably correctly into actual values.
The instantaneous power of the converter can be determined from voltages and currents. A suitable filter can be used to obtain the power component which is largely converted into mechanical power in the motor (the stator and rotor losses must be deducted). This power P then corresponds to the torque M multiplied by the angular velocity 2π·nmech as per the formula:
  M  ≈      P          2      ⁢              π        ·                  n          mech                    
The mechanical torque can in principle also be estimated in accordance with the following formula:
      M    ~    ≈            P      δ              2      ⁢              π        ·        f            
It is simpler to obtain the torque using this second equation, since the rotor power input Pδ can be determined more easily (average active power of the converter less the stator losses). Instead of the mechanical speed nmech the electrical output frequency f of the converter can be used. This is always present as a target value in the controller.
The motor losses (stator and rotors) can likewise be derived from the measured variables using corresponding models. The energy component in the stator inductances of the motor can thus be taken into account.
Compensation for an output voltage deviation in the case of a power converter is known from the publication GB 2 440 559 B. In this case an integrated voltage measurement of the three output voltages is performed. This is done with a Σ-Δ AD converter.
It would be desirable and advantageous to provide an improved method for estimating the torque of a drive of a vehicle more reliably